Re: [whatwg/url] Punycode behavior for labels exceeding DNS length is ill-defined (Issue #824)

I think conceptually the idea is that we don't want to couple the hostnames of special URLs _too tightly_ with DNS, although I don't think it is well defined (long-standing issue https://github.com/whatwg/url/issues/397).

That said, a length limit on the encode function to prevent overflow is a practical necessity. My library uses 32-bit integers, so to prevent overflow we refuse to encode input sequences longer than 3854 scalars. That's not even for DoS protection; it's just for correctness.

https://github.com/karwa/swift-url/blob/01ad5a103d14839a68c55ee556513e5939008e9e/Sources/IDNA/Punycode.swift#L185-L202

I wouldn't necessarily mind a tighter bound to keep the n^2 encoding algorithm under control. I'm not sure it makes sense to use length limits from DNS, though.

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